Not applicable.
Not applicable.
This invention is in the field of telecommunications, and is more specifically directed to improved upstream communications from a client modem to a telephone network central office over conventional analog telephone lines.
With the recent explosion in Internet usage by both business and consumers, the use of dial-up modem communication between personal computer users and Internet service providers (ISPs) over conventional telephone networks, such as the Public Switched Telephone Network (PSTN) in the United States, has become widespread. Important advances have been made in recent years that have greatly increased the data rate at which communications may be carried out over the existing telephone network, realized to a large degree over twisted-pair wires. These important advances have also greatly reduced the cost of equipment required for such high data rate communication, as well as the cost of providing communications services.
These advances have also improved the capacity of telephone networks to carry higher volumes of voice and data traffic. One such advance is the advent of digital communications over the telephone network among the various xe2x80x9ccentral officesxe2x80x9d, generally carried out over fiber-optic cable. One well-known technique used to implement such digital communications utilizes coder/decoder functions (xe2x80x9ccodecsxe2x80x9d) at the central offices, as will now be described.
FIG. 1 illustrates a simple example of communication between a pair of telephones U over a conventional telephone network. In this example, telephones U1 and U2 are remote from one another, such that telephone U1 is associated with central office CO1, and telephone U2 is associated with central office CO2. As is well known in the art, the term xe2x80x9ccentral officexe2x80x9d refers to a field location of the local telephone company at which switching and other telephone network management functions take place. Communications between telephones U1 and telephone U2, in this conventional example, will be carried out by way of analog connections between telephones U1, U2 and their respective central offices CO1, CO2; the analog communication system between telephones U and central offices CO are referred to as xe2x80x9csubscriber loopsxe2x80x9d. Codecs 21, 22 are provided within central offices CO1 and CO2, respectively, to convert the analog signals received from their respective telephones U1, U2 into digital signals for communication over PSTN 4, and vice versa. As illustrated in FIG. 1, each codec 2 includes analog-to-digital converter (A/D) 6, for converting analog signals traveling from its associated telephone U into digital signals to be sent over PSTN 4; conversely, each codec 2 also includes digital-to-analog converter (D/A) 7, for converting digital signals received from PSTN 4 into analog signals for communication to its associated telephone U. The digital communication over PSTN 4 may be carried out over fiber optic facilities or other high speed communications trunks, according to conventional standards such as DSO, SONET, and the like. As a result of the conversion of the analog voice signals into digital bitstreams, the volume of telephone traffic carried by PSTN 4 can be quite large.
Each of codecs 2 include an associated A/D 6, as noted above. According to conventional implementations, each A/D 6 converts the incoming analog signal into a stream of digital values by way of sampling. Typically, the sampling rate of A/D 6 is 8 kHz. As is fundamental for A/D conversion, each sample of the analog signal is converted into a digital value that most closely approximates its amplitude. In modern voice telephone networks, the conversion carried out by A/D 6 typically follows a xcexc-law estimation, rather than a linear mapping of analog to digital values, in order to provide a pleasant voice signal with relatively accurate dynamic range. In any event, error results from the approximation that is necessarily in A/D conversion, amounting to the difference between the true amplitude of the sampled analog signal and its digital approximation; such error is commonly referred to as xe2x80x9cquantization errorxe2x80x9d. The amount of quantization error, or conversely the precision of the conversion, depends directly upon the number of bits used to express the digital value. Current standards for voice communication call for an eight-bit analog-to-digital conversion at the central office, such that each analog sample amplitude is assigned to one of 256 possible values. As such, quantization error may be significant, especially at high data rates.
As noted above, modem communications among computing devices has now become widely popular, especially with the widespread use of the Internet by business and individuals. As is fundamental in the art, a modem (shortened from xe2x80x9cmodulator/demodulatorxe2x80x9d) is a device that converts signals to be communicated from the computer over the telephone network from a digital bitstream into an analog signal within the voice band (and vice versa), so that data communications can be readily carried out over the same telephone network used for voice communications. FIG. 1 illustrates a typical home computer C1 having a modem M1 that is connected into the telephone network, for example at the same location at which telephone U1 is connected. As illustrated in FIG. 1, digital communications are of course carried out between computer C1 and modem M1, but communications between modem M1 and central office CO1 are analog, just as are the voice communications between telephone U1 and central office CO1. As such, communications from computer C1 and PSTN 4 begin as digital data, are converted into analog signals by modem M1, and are converted back into digital form by A/D 61 in central office CO1.
In the arrangement of FIG. 1, codec 21 necessarily inserts error into the communication of digital data from computer C1 to PSTN 4. This error is directly due to the quantization error of A/D 61. Because of the approximation carried out by A/D 61, its digital output will not always match the corresponding digital value presented by computer C1 to modem M1 for conversion into the analog domain. The presence of this quantization error at central office codecs is currently the limiting factor in the data rates at which modem communications may be carried out over modern telephone networks, especially considering that Federal Communications Commission rules limit the maximum power of telephone communications.
By way of further background, the V.34 standard for modem communications at data rates of up to 33.6 kbps utilizes a precoder in the transmitting modem. The modulation used according to this standard is quadrature amplitude modulation (QAM). This precoder is intended to implement the feedback coefficients obtained in decision feedback equalization for the communications channel, partially compensating for channel distortion; the remainder of the distortion is compensated by a linear equalizer at the receiving modem.
Recently, it has been observed that the bulk of modem communications are telephone companies carry out digital communications between the PSTN and the ISPs, whether or not such communications travel through a central office. An example of this conventional arrangement, for a single remote computer C1, is shown in FIG. 2. In this example, computer C1 communicates digitally with modem M1 as before; modem M1 in turn communicates with central office CO1 using analog signals. Central office CO1 includes codec 21 as before, with A/D 61, converting the analog signals from modem M1 into digital signals for communication over PSTN 4; codec 21 also includes D/A 71 for converting digital traffic into analog signals, for communication to modem M1. As shown in FIG. 2, however, ISP 10 communicates digitally with PSTN 4 (perhaps through a central office, not shown in FIG. 2), and as such no analog-to-digital conversion of signals from PSTN 4 is required.
In the conventional arrangement of FIG. 2, data communicated from ISP 10 to computer C1 is not converted from analog into digital by a central office codec in order to be communicated over the telephone network. The only conversion from analog into digital for such xe2x80x9cdownstreamxe2x80x9d data is at modem M1; however, the precision with which modem M1 effects this conversion may be very high, as this conversion is not subject to the voice communications standards (e.g., 8 kHz sampling rate, 8-bit xcexclaw conversion) as are central office codecs. As a result, quantization error is not a data-rate-limiting factor in these downstream communications. In contrast, upstream communications from computer C1 to ISP 10 is converted from analog to digital by A/D 61 in codec 21 of central office CO1. This conversion, as noted above, can involve significant quantization error, which limits the maximum data rate for such upstream communications.
Recent modem communications standards have recognized the difference between upstream and downstream modem communications. According to the current V.90 recommendation, downstream (ISP-to-client) communications may now be carried out, over conventional telephone network wiring, at a data rate up to as high as 56 kbps, using pulse code modulation (PCM) of the digital signal. However, the quantization noise in the upstream (client-to-ISP) path caused by the central office A/D conversion, currently limits the upstream data rate to 33.6 kbps.
By way of further background, U.S. Pat. No. 5,528,625 describes a technique by way of which the effects of quantization noise from central office codec A/D conversion may be limited, thus permitting higher data rate upstream communications. As noted therein, the effects of quantization distortion, or quantization noise, may be reduced by utilizing the xcexclaw quantization levels themselves as the digital channel symbol xe2x80x9calphabetxe2x80x9d of each of multiple communications legs at the modem. Each of the communications legs are separately equalized to precompensate for the-response of the subscriber loop. As a result, the digital-to-analog modulation carried out by the modem generates analog signals that are effectively at the xcexclaw quantization levels of the A/D converter in the-central office codec, pre-compensated for channel distortion.
Another issue in modem communications is the less than optimal bandwidth of bandpass filters in conventional codecs. While the A/D converter in conventional codecs sample at 8 kHz, the subscriber loops are typically bandlimited, by codec bandpass filters, to a range of approximately 3.3 kHz over which the frequency response is usable. This reduces the effective maximum symbol rate (assuming no quantization error) to approximately 6 kHz which limits the data rate to 48 kbps, for eight-bit encoding.
The above-referenced U.S. Pat. No. 5,528,625 describes the separate equalization of each of multiple transmission legs to eliminate quantization noise. In effect, multiple transmit and receive filters are defined, with the number of such channels corresponding to the number of valid independent pulse-code modulation (PCM) symbols to be transmitted per eight symbol frame. Because of the limitation resulting from the bandlimiting of the subscriber loop, at most six of eight symbols carry valid data, and thus six transmit and receive filters are defined. While this approach appears to have merit in improving the data rate of conventional modems, its implementation appears to be relatively complex and thus quite costly.
It is therefore an object of the present invention to provide a modem and method of operating the same that provides improved data rate upstream communications.
It is a further object of the present invention to provide such a modem and method that can achieve such improved data rate over conventional twisted pair telephone networks.
It is a further object of the present invention to provide such a modem and method that may be implemented at relatively low cost.
It is a further object of the present invention to provide such a modem and method that may be implemented in a computationally efficient manner.
It is still a further object of the present invention to provide such a modem and method that would require minimal changes to existing communications standards.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.
The present invention may be implemented by way of a pre-equalizer arrangement in the transmission of data by client-side modems. As part of the connection protocol, the client modem transmits a training sequence that is received by the central office digital modem; the digital modem determines the pre-equalizer coefficients according to channel effects upon the training sequence, preferably according to an error minimization that considers a maximum average voiceband power limit. These coefficients are forwarded to the client modem, which inserts the coefficients into a digital pre-equalizer filter function.